Silvering Plastic Scintillators

[KailieNadine]

I am experimenting with some Cosmic Watch v3x muon detectors and right now I have the scintillators (BC408) wrapped in foil as recommended. I am wondering though, would there be an advantage to silvering the scintillators (either with Tollens' reagent or CBD/PVD) or using Rad-Film instead of foil?

Thanks,
Kailie

[iRad]

Kailie - Great question...
The purpose of the foil wrap recommended in the CosmicWatch project is to increase internal reflection and block external light. Foil is a simple, quick and effective way to accomplish this, but not necessarily the best reflector of the internal scintillations. Foil also effectively serves the second purpose of blocking all light from entering the scintillator/SiPM assembly. Metalized foils and other mirrored materials (including Rad-Film), or silver surface coatings, may be great for reflecting visible light but do not best reflect the UV frequency light emitted by most scintillators. Bright white materials seem to do a better job with this.

White PTFE tape (often used as pipe thread sealing tape) is suitable and often used as a very good scintillator reflector. A bright white paint also works well, especially if you can find a paint specifically formulated for high percentage light reflection. A good flat bright white primer paint designed for plastic use works well enough. Of course, with these types of semi-transparent reflectors, additional opaquing material or an opaque housing must be used to completely block all external light from entering the detector assembly.

Cheers, Tom

[KailieNadine]

Hi Tom,
Thanks for the response! I hadn’t even considered that, but it makes a lot of sense. I just finished polishing two more scintillators, so I’ll wrap those in PTFE instead.

One more question, do you think that drilling the scintillators and attaching the SiPM board with screws affects performance at all? Would it be better to couple the SiPM to the scintillator some other way that doesn’t involve drilling into the actual block?

[iRad]

Kailie - Drilling would cause some interference with internal reflection, but I think the impact would be small. Using a good permanently clear epoxy for mounting purposes is an alternative, but that may have its own issues. If something goes wrong, it could be a mess to correct. Compression wrapping the detector assembly with black electrical tape might be another alternative, especially if the scintillator was painted white. This is often done when building scintillator detectors using PMTs.

BTW: I failed to mention previously that when using paint as a reflector, it should be sprayed onto the surface of the scintillator for best reflectivity. Brushed paint does not work nearly as well as a reflector, and aluminum foil would probably work better than brushed paint. Of course, PTFE tape would be much preferable to both methods. Even better than all but the best reflective paint sprayed onto the scintillator.

Cheers, Tom

[Sesselmann]

Place the wired SiPM against the scintillation block with some optical silicone grease and wrap it tightly with PTFE tape, make sure you have multiple layers if PTFE tape.The tape not sticky but once you have a few turns it will hold. Then package the whole thing in aluminum foil sticky tape making sure that no light can get in, use a double foil overlap where the wires exit.

[iRad]

Place the wired SiPM against the scintillation block with some optical silicone grease and wrap it tightly with PTFE tape, make sure you have multiple layers if PTFE tape. The tape not sticky but once you have a few turns it will hold. Then package the whole thing in aluminum foil sticky tape making sure that no light can get in, use a double foil overlap where the wires exit.

Steven is correct, this is probably the best way to go. The "aluminum foil sticky tape" is sold in hardware stores (Home Depot) and often used for sealing AC ductwork. Not very good for reflectivity (you have the PTFE for that), but excellent for making a compact light tight enclosure.

[KailieNadine]

Place the wired SiPM against the scintillation block with some optical silicone grease and wrap it tightly with PTFE tape, make sure you have multiple layers if PTFE tape.The tape not sticky but once you have a few turns it will hold. Then package the whole thing in aluminum foil sticky tape making sure that no light can get in, use a double foil overlap where the wires exit.

The current batch of SiPMs I have are already soldered to small PCBs (which will then be plugged into the main PCB), although I think I can still tape the small PCB to the scintillator this way for now. Is there any disadvantage to soldering wires to the PCB vs soldering it directly to a board? Wires would certainly allow more design flexibility, which would be great!

[KailieNadine]

Place the wired SiPM against the scintillation block with some optical silicone grease and wrap it tightly with PTFE tape, make sure you have multiple layers if PTFE tape. The tape not sticky but once you have a few turns it will hold. Then package the whole thing in aluminum foil sticky tape making sure that no light can get in, use a double foil overlap where the wires exit.

Steven is correct, this is probably the best way to go. The "aluminum foil sticky tape" is sold in hardware stores (Home Depot) and often used for sealing AC ductwork. Not very good for reflectivity (you have the PTFE for that), but excellent for making a compact light tight enclosure.

Thank you! I have picked some up today and will be giving it a go.